This invention relates to an improved method for preparing light-absorbing polymeric compositions, which are useful as powders or pellets for incorporation into a variety of thermoplastic resins such as cellulose esters, polyesters, polyolefins, polycarbonates, polyamides, etc. by conventional melt or solution blending techniques. The colored thermoplastic resins thus produced have good clarity, good color development, excellent fastness to light and are useful for a variety of end uses where nonhazardous, nonmigrating, or nonextractable colorants are needed.
It is well-known that thermoplastic polymers may be colored by adding pigments or solvent dyes (e.g., see Thomas G. Weber, Editor, Coloring of Plastics, John Wiley & Sons, New York, 1979). The use of pigments, however, is accompanied by undesirable properties such as opacity, dullness of color, low tintorial strength, etc. Also, difficulties in uniformly blending the insoluble pigments with the thermoplastic resin are often encountered. Also useful for coloring thermoplastic polymers are the solvent dyes (K. Venkataraman, Editor, The Chemistry of Synthetic Dyes, Vol. 8, Academic Press, New York, 1978, pp. 81-131), which provide compositions having improved clarity, brightness in hue and high tinctorial strength, but which may lead to dye migration, extraction, etc. from the colored thermoplastic polymer. These problems are of particular concern when solvent dyes are used to color flexible polymers such as polyvinyl chloride, polyethylene and polypropylene which have low glass transition temperatures.
Plastics, paints, printing inks, rubber, cosmetics, and similar materials are typically colored by organic pigments when superior brilliance and tinctorial strength are important. Toxicity considerations have presented chronic problems relative to the use of organic pigments since some have been shown to be potential carcinogens and to cause contact dermatitis.
Plastics are also colored by using color concentrates consisting of physical admixtures of polymers and colorants (usually solvent dyes). However, the use of such physical admixtures to color polymeric materials such as polyester, e.g., poly(ethylene terephthalate) and blends thereof, present a number of problems, including:
Colorant migration during drying of the colored polyester pellets. PA1 Colorant migration during extrusion and colorant accumulation on dies which can cause shutdowns for clean-up. Such colorant migration and accumulation result in time consuming and difficult clean-up, particularly when a polymer of another color is subsequently processed on the same equipment. PA1 Colorants may not mix well, for example, when using two or more color cencentrates to obtain a particular shade. PA1 Colorants may diffuse or exude during storage and use of the colored polymeric material.
The colored polymeric compositions which are prepared by the process of this invention eliminate or minimize the aforementioned problems associated with the use of conventional dyes and pigments.